The present invention relates to a multi-layer shielded wire with excellent electromagnetic shield capability, which is mainly used in a vehicle.
In a vehicle, a shielded wire having an electromagnetic shield layer composed of a metal conductor is used as a transmission line of a radio frequency (RF) signal, an image signal or a communication signal received by an antenna.
The shield wire is formed by covering an inner conductor (one core or multicore) with an insulating layer, covering the insulating layer with an outer conductor, and providing a protective layer such as vinyl chloride (PVC) on an outermost layer as a protective material.
The outer conductor is mainly composed of a metal foil or a braided wire. The metal foil which is formed by attaching several μm of aluminum or copper on the surface a thin plastic film such as polyethylene and has a film shape is generally used. The braided wire which is formed by braiding a plurality of copper thin lines (wires) is generally used.
The metal foil and the braided wire are different from each other in the frequency characteristics of the shield capability. If the outer conductor is composed of one layer, although changed according to the condition, if the braided wire is used in a frequency band of 100 MHz or less and the metal foil is used in a frequency band of more than 100 MHz, the capability is high. The metal foil and the braided wire are properly selected according to the use purpose of the electric wire from the viewpoint of terminal machining or mechanical strength as well as shield capability.
From the same reason, if the use purpose is not achieved by one layer, the metal foil or the braided wire may overlap by two layers or more or a combination of the metal foil and the braided wire may be used. In particular, in a high frequency band of 100 MHz or more, since it is difficult to obtain the shield effect by one layer compared with a frequency band of less than 100 MHZ, a multilayer structure is generally used.
If the outer conductor is composed of two layers or more, there is a case of inserting an insulating layer between the layers and a case of electrically contacting electromagnetic conductors without inserting an insulating layer. The former case may be called two layers and the latter case may be called two folds. Even in the two layers, the terminal may be short-circuited when the electric wire terminal is machined.
This type of related multilayer shielded wire is, for example, disclosed in Patent Document 1 or Patent Document 2.
FIGS. 8A and 8B show a related configuration example of a two-layer shielded wire. The two-layer shielded wire 100 is formed by covering an outer circumference of an one-core inner conductor 111 with a first insulating layer (dielectric) 112, sequentially covering the first insulating layer with a first outer conductor (electromagnetic shield layer) 113, a second insulating layer (dielectric layer) 114 and a second outer conductor (electromagnetic shield layer) 115, and providing a protective layer 116 on an outermost layer.
A shielded wire 120 shown in FIG. 9 is formed by directly contacting outer conductors 113 and 115 without inserting an insulating layer between the first outer conductor 113 and the second outer conductor 115.
[Patent Document 1] JP-A-2006-173044
[Patent Document 2] JP-A-2003-229028
In the related multilayer shielded wire, if shield capability is desired to be increased, manufacturing cost is increased, a weight is increased or the diameter of the electric wire is increased as the number of layers is increased.